Cell growth and differentiation occur by a signal transmission mechanism in which extracellular signals act on the genetic program of cells. The majority of genes in which transcription is initially modified by signal transmission are genes that code for transcription regulation factors. Those substances having molecules that are responsible for this type of signal transmission are called ligands.
Ligands are molecules that specifically bind with protein, and the proteins that bind with ligands are referred to as ligand-binding proteins. Examples of ligands include various drugs, nucleotides and metal ions, while examples of ligand-binding proteins include proteins generically referred to as receptors or enzymes and various other types of biologically active proteins.
Ligands are believed to be involved in the signal transmission of various biological activities as a result of binding with ligand-binding proteins.
Metal ions in particular are known to be ligands that fulfill important roles in the body. For example, zinc ions have been clearly shown to bind with numerous transcription regulatory factors and act on the expression of their functions. In addition, transcription regulatory factors are known to be involved in generation, differentiation, inflammation, cell growth and so forth. However, transcription regulatory factors are only present in the body in trace amounts, thus making their purification and the analysis of their molecular structure difficult.
Some transcription regulatory factors are known to have a zinc finger structure. Attar, R. M. et al. (Mol. Cell. Biol. (1992) 12, 2432-2443) reported the cloning of a gene coding for a protein having a zinc finger structure that binds with a serum response factor by using a serum response factor DNA probe.
Methods which utilize the homology of an amino acid or nucleotide sequence based on its characteristic basic structure are not considered to be able to be used for cloning of genes that code for zinc finger proteins. Although the majority of zinc finger proteins that have already been cloned were acquired based on the partial amino acid sequence of an intranuclear receptor or other purified protein, other known zinc finger proteins include those identified as proteins that bind to the transcription regulatory region, such as the GATA transcription factor group (Tsai, S. F., et al., Nature (1989) 339, 446-451), and those determined to be zinc finger proteins as a result of structural analysis of the chromosome translocation site of leukemia cells, such as Evans, T. et al. (Cell (1989) 58, 877-855) and Ttg-1 gene (Mcguire, E. A. et al., Mol. Cell. Biol. (1989) 9, 2124-2132). Similarly, the PML gene was also newly identified from analysis of the chromosome translocation site of APL, while PML-RAR, which is the product of fusion by chromosome translocation with the zinc finger, retinoic acid receptor (RAR), is thought to be intimately involved in transcription regulation of leukemia cells (Kakizuka, A. et al., Cell (1991) 66, 663-674, de The, H. et al., Cell (1991) 66, 675-684). In this manner, although zinc finger proteins have important functions, there are as of yet no direct methods having high specificity that involve binding of ligand, namely zinc ion, without limiting the target DNA.